regenerative heating of the feed water (rhfw) · temperature on the inlet of steam turbine is...

REGENERATIVE HEATING OF THE

FEED WATER (RHFW)

TOMSK POLYTECHNIC UNIVERSITY

MAIN FEATURES

Theoretical cycle with heat regeneration

Real cycle with heat regeneration

Effect of RHFW on the efficiency of NPP

Distribution of regenerative bleed into turbine

Steam and drainage cooling

EFFECT OF FEED WATER

TEMPERATURE ON EFFICIENCY

To improve efficiency of NPP the average temperature of heat

supplied to cycle should be increased. When ability to increase

temperature on the inlet of steam turbine is depleted this could

be reached by increasing the temperature of the feed water on

the inlet of steam generator.

The most efficient method to increase temperature on the inlet

of steam generator is application of regenerative heating of feed

water.

REGENERATIVE HEATING OF THE

FEED WATER (RHFW)

Regenerative heating of the feed

water (RHFW) – application of

heat of working fluid, which have

conducted mechanical work in

turbine, on the different part of the

cycle.

Effect of RHFW:

Decreasing of the thermodynamic

irreversibility of heating on the

line k’-a’

Decreasing of heat losses into

cold source (condenser)

THEORETICAL CYCLE WITH REGENERATIVE

SCHEME OF THEORETICAL CYCLE

WITH REGENERATIVE HEATING

FINITE THEORETICAL CYCLE WITH

REGENERATIVE HEATING

Finite theoretical regenerative cycle:

Infinite amount of regenerative bleeds;

Infinitively low heat drops at each stage.

Impossibility of theoretical regenerative cycle:

Unacceptable humidity in the end of expansion process;

Large dimensions of heaters, bleed cross-section area and

pipelines;

High pressure losses.

THREE STAGE RHFW OF THE

OPEN TYPE

One stage of RHFW

includes:

Steam bleed;

Heater;

Steam pipeline;

Pump (optional);

Drain line (optional).

SCHEME AND CYCLE OF TWO STAGE

RHFW OF THE CLOSED TYPE

WORK OF TURBINE IN SCHEME

WITH TWO STAGE RHFW

Theoretical work:

lt=lI+lII+lk=(h0-h1t)+(1-α1)(h1t-h2t)+(1-α1-α2) (h2t-hkt)=h0- -α1 h1t-α2 h2t-αk hkt

Real work:

lt=lI+lII+lk=(h0-h1)+(1-α1)(h1-h2)+(1-α1-α2) (h2-hk) =h0- -α1 h1-α2 h2-αk hk

PARAMETERS OF NPP IN SCHEME

WITH N-STAGED RHFW

Effective heat drop of turbine with nth stages:

By substituting we get:

0

1

np p

i i j j k k

j

l H h h h

0

1

np

i k j j k

j

H h h h h

1

1n

k j

j

PARAMETERS OF NPP IN SCHEME

WITH N-STAGED RHFW

Coefficient of turbine power decreasing:

Turbine power using values of yj:

0

1 1

1n n

p

i k j j k i j j

j j

H h h h h H y

0

j k

j

k

h hy

h h

EFFICIENCY OF NPP IN SCHEME

WITH N-STAGED RHFW

Efficiency of theoretical NPP with RHFW:

Efficiency of real NPP with RHFW :

0 010 0

1 0 0

np p

k kt j jtjp

t p

fw fw

h h h hH H

q h h h h

0 01

1 0 0

np p

k k j jjp i it p

fw fw

h h h hH H

q h h h h

EFFECT OF RHFW ON NPP

EFFICIENCY

Energy coefficient with regeneration:

Efficiency of theoretical NPP with RHFW:

01

0

n

j jjp

k k

h hA

h h

1

1

preg p

i i p p

i

A

A

EFFECT OF NUMBER OF RHFW

STAGES ON NPP EFFICIENCY

Increasing z results into increasing efficiency.

Maximal efficiency is obtained at optimal temperature of feed water.

Optimal temperature of feed water increases with z.

With increasing z the thermal efficiency decreases.

On practice the 7-8 stages of RHFW is applied. I allows to increase efficiency by 15-17 %.

EFFECT OF NUMBER OF RHFW

STAGES ON NPP EFFICIENCY

Power of turbine of NPP with RHFW:

Steam flow rate on turbine with RHFW:

Note. Steam flow rate in NPP with RHFW is larger in comparison with NPP without regeneration.

1

1n

p

i i i j j

j

N G H G H y

1

1

i

n

i j j

j

NG

H y

1

11

1p n

j j

j

k

y

CHOICE OF NUMBER OF BLEEDS

AND PRESSURE

To reach maximal thermal efficiency of NPP the

different methods could be applied.

EVEN DISTRIBUTION OF HEATING

Assuming the heat taken from 1 kg of steam from

one heater to another to be constant the

maximal efficiency could be obtained at even

distribution of enthalpy increasing.

EVEN DISTRIBUTION OF FEED

WATER ENTROPY

EVEN TEMPERATURE RELATION

INDIFFERENT POINT

REGENERATION IN NPP WITH HEAT

SUPPLY

CHOICE OF OPTIMAL TEMPERATURE

OF FEED WATER

Choice of optimal feed water temperature is complex technic-economic task.

Factors affecting regeneration parameters:

Specific power of equipment;

Initial parameters of steam;

Cost of materials and fuel.

Taking into account economic factors the amount of regenerative heaters is chosen to be 7-9 in Russia and up to 11 abroad. The technic-economic optimal temperature of feed water is lower than thermodynamic optimal.

COMPARISON OF SURFACE-TYPE

AND MIXING HEATERS

COMPARISON OF SURFACE-TYPE

AND MIXING HEATERS

MIXING HEATERS

Advantages:

Simplicity;

Lower cost;

Higher reliability;

Simplicity of exploitation and repairs;

Lesser concentration of metals and oxides.

NO UNDERHEATING.

Disadvantages:

Need for additional pumps;

Need for additional protection from droplets entering turbine (hard to realize at high pressures).

SCHEMES OF MIXING HEATERS

The heaters of mixing type is applied for the first

two steps of regenerative heating.

SURFACE HEATERS

Advantages:

Independence of steam and water pressures;

Possibility to apply one pump.

Disadvantages:

Complexity;

Lesser reliability;

Higher costs;

UNDERHEATING.

SCHEMES OF SURFACE HEATERS

The heaters of mixing type is applied for the first

two steps of regenerative heating.

COMPARISON OF SCHEMES

Scheme Advantages Disadvantages

A Maximal efficiency Many pumps pumping water with

high temperature

B Simplicity

No pumps needed

Steam condensate push out

steam from next heaters

Additional losses into condenser

C Intermediate solution between schemes A and B

Aim: decrease “pushing out”

Main feature: temperature of condensate in drainage

cooler is higher by 10-15 °C in comparison with feed water temperature.

D

STEAM COOLER

Temperature of steam after steam cooler is higher by 10-15 °C than saturation temperature.

Application of steam cooler gives:

higher feed water temperatures on the outlet;

increased efficiency of regeneration;

increased bleed steam consumption.

DRAINAGE/CONDENSATE COOLER

THANK YOU FOR YOUR ATTENTION